The FSharp.Core should be retrieved from the hosting environment

 - fixes #156 and https://github.com/fsharp/FAKE/issues/486

I also fixes FSharp.Core.dll resolution in FSharp.Formatting.

DISCLAIMER:

![uvj9hf7](https://cloud.githubusercontent.com/assets/57396/3441652/84a744f6-0107-11e4-8f63-72511e0d6b81.gif)